In a first main aspect the invention relates particularly to an active noise cancelling system which detects ambient noise and applies electro-acoustic processing thereto to produce an acoustic signal for cancelling out the ambient noise. The active noise cancelling system may be used to cancel all noise but for an audio signal which is desired to be heard by the user.
In a second main aspect the invention relates particularly to a system for electronically cancelling noise input to a user microphone, by utilizing known characteristics of speech signal and ambient noise.
The first main aspect of the invention can be used in concert with the second main aspect in a communication system (e.g. telephone network) to provide cancellation of both ambient noise in the vicinity of the earphone outputting the received audio signal (e.g. telephone earpiece) and electronic cancellation of ambient noise being received at an input microphone to the communication system (e.g. telephone mouthpiece).
Other aspects of the invention relate respectively to speaker stability compensation in a closed loop feedback system for driving a loudspeaker, an adaptive control device and method for adjusting parameters of a closed loop noise cancelling system in order to optimize noise cancellation a method and apparatus for providing a power supply for audio signal processing equipment, a method and apparatus for minimizing power consumption in sound processing equipment (e.g. noise cancellation and/or audio signal processing equipment), and provision of an accoustic waveguide to enhance noise cancelling performance of a noise cancelling system.
There are many applications where an efficient noise cancelling system is desirable. Such applications include, for example, "ear protection" in industry, noise cancellation for communications headsets in aircraft, noise cancellation in general communications systems such as telephone systems, etc.
Previous approaches to the problems of noise include the provision of ear protectors to try and physically block out ambient noise. This is an approach used particularly in noisy industrial environments where a worker may be exposed to high noise levels which could result in damaged hearing after prolonged exposure. Unfortunately, such ear protectors tend to be "leaky" and therefore still let some noise through, particularly at lower frequencies as well as being generally bulky and uncomfortable to wear.
It has also been proposed to cancel noise in a known predefined space by sensing the noise with sensors placed at known parts in the area and producing an audio signal of the same magnitude and 180 degrees out of phase with the noise to cancel the noise (see in particular patent publication WO89/11841). However, the processing involved in such a noise cancellation system is extremely complex and can only be designed to cancel synchronous, repetitive background noise and not intermittent noise which does not occur at regular intervals. This intermittent noise will still be heard by the user. Another example of disclosure relating to the cancellation of repetitive noise is given in Australian patent application number AU 85255/82.
It is known to provide Active Noise Cancelling (ANC) by providing a microphone to sense ambient noise and an earphone placed in the vicinity of the microphone to produce a noise cancelling acoustic signal. However, such a noise cancelling system is only effective over very narrow frequency ranges. Noise falling outside the effective frequency range of the system will still be a problem. An original patent to Olsen (1953) discloses such a system. A microphone placed near the user's ear detects noise and is used to electrically drive an earphone loudspeaker. By increasing the electronic gain in the cancellation loop a frequency is found where the loudspeaker audio signal is equal in magnitude and opposite in phase with the external noise pressure wave at the listener's ear, causing attentuation of the noise. This electromechanical phenomenom is commonly known as "Active Noise Control" (ANC). Unfortunately, it is only effective over a narrow frequency range, as discussed above. This range is too narrow to cause sufficient attentuation of noise over a satisfactory range for most practical applications.
There is therefore a need for an active noise cancelling system that is effective over a wide range for practical noise cancelling applications.
The microphone/speaker ANC arrangement forms a closed loop system which will have particular electro-acoustic transfer function characteristics.
For frequencies below the first flexural (acousto-mechanical) resonance of the speaker or microphone we have found that the open loop electro-acoustic transfer function of any microphone/speaker arrangement is predominantly determined by the loudspeaker suspension resonance characteristic. With knowledge of this it becomes possible to calculate, i.e., mathematically model the electro-acoustic transfer function of any particular microphone/speaker ANC arrangement for at least the frequencies which are below the first flexural resonance of the speaker or microphone, which tend to be the frequencies, for most standard microphones/speakers, which one is interested in with regard to audio noise cancelling.
In order to provide wide frequency band noise cancellation a closed loop electro-acoustic transfer function of negative unity is required for the microphone/speaker ANC closed loop system so that the audio signal produced at the speaker is of the same magnitude and 180 degrees out of phase with the noise signal for at least the wide frequency range within which noise cancellation is required. In previous systems such a negative unity transfer function is not obtained over the desired range.